1. Field of the Invention
The present invention relates to an optical recording medium for recording and/or reproduction of information with light, particularly to a recording medium having excellent durability. The present invention also relates to a process for producing the optical recording medium.
2. Related Background Art
Various types of optical recording media have been commercialized and are now being studied and developed for use for optical information recording and/or reproduction apparatus such as compact disks, additional-writing type disks, and optical disks employing an erasable magnetooptical material or a phase transition type material. Lately, portable optical recording media such as a rectangular card-shaped optical recording medium (hereinafter referred to as an "optical card") in a size of 85 mm.times.54 mm are attracting attention.
A conventional optical recording medium, for example, has a construction as shown in FIG. 4 in which a recording layer 32 is formed on a substrate 31, and thereon an adhesion layer 33 is laminated, and further thereon a protection layer 34 is laminated with interposition of an adhesion layer 33. In such a construction, an end portion of the recording layer is bare and is exposed to the outside at the side end face of the optical recording medium as shown in FIG. 4. Therefore, the recording layer is liable to deteriorate owing to penetration of moisture or oxygen from the bare portion of the recording layer into the interior thereof, disadvantageously. Although the substrate 31 and the protecting layer 34 are bonded with interposition of the recording layer 32, the adhesion strength is not strong enough, and the recording layer is liable to be separated from the substrate. If separation occurs in this portion, the surface also of the recording layer becomes bare and the deterioration of the recording layer will be accelerated.
To offset such disadvantages, U.S. Pat. No. 4,432,002 discloses an optical recording medium in which the recording layer 32 is sealed by the adhesion layer 33 without bareness of the recording layer at the side end face of the recording medium as shown in FIG. 5 to stabilize the recording layer. In such a recording medium, the adhesion layer itself is required to have little effect on the recording layer for protecting the recording layer against the external environment and giving satisfactory adhesion.
In the case of personally carried optical recording media such as an optical card, it should be taken into account that the optical card is kept under severe conditions than optical disks for use for filing systems in offices. For example, the tentative standard for optical cards laid down by Japan Business Machine Makers Association proposes to impart chemical resistance to an optical recording medium such that the function of the optical recording medium is not affected by a variety of chemicals (e.g., acetic acid, etc.). However, a satisfactory adhesive has not been found which has little affect on the recording medium, while having sufficient chemical resistance, and exhibiting a high level of adhesion performance.
In the construction as shown in FIG. 5, the recording layer needs to be formed in a pattern on the substrate in order that the recording layer is not bare at the end face.
The process of forming a recording layer of an optical recording medium is selected depending on the material used for the recording medium. For example, vacuum film-forming methods including vapor deposition and sputtering are employed in the case where the recording medium is formed from an inorganic material such as a metal, e.g., like aluminum, and an amorphous film of a carcogenide, e.g., bismuth, tellurium oxide, etc. In the case where the recording layer is formed from organic pigment, wet coating methods of application of a solution or a dispersion of an organic pigment in water or a solvent are employed since such methods enable formation of the recording layer at a low cost at high productivity. In any method, formation of the recording layer in a pattern involves problems in improving productivity.
When the patterned recording layer is formed by a vacuum film-forming method, a mask for covering the portion other than the recording layer region has to be placed in a precise position. When the patterned recording layer is formed by a wet coating method, a technique is required for pattern printing by use of a screen plate or a gravure plate. When a preformat such as track grooves and prepits is formed on a substrate for information recording, the position of the recording layer formation has to be registered precisely relative to the preformat.
The wet coating method of film formation inevitably gives thickness change at an end portion of the recording layer (hereinafter this portion is referred to as a "non-uniform thickness region" of a recording layer) as shown in FIG. 6 owing to a transition phenomenon. Furthermore, the reflectivity of the recording layer composed of an organic pigment varies remarkably at a thin region as shown in FIG. 7. Therefore, the end portion of the recording layer cannot be used practically as a region 63 for recording and reproduction of information (hereinafter referred to as a "recording-reproduction region") to record and/or reproduce information stably. The uselessness of the recording layer end region as the recording-reproduction region will further decrease disadvantageously the recording capacity of the optical recording layer in addition to the decrease caused by the formation of a margin region 62 at the end portion for preventing the recording layer from bareness.
This problem is of great technical importance for optical recording mediums like optical cards which are carried by individual persons and limited in external shape and size.
The present invention was made to solve the above problems.